There are a number of scanning systems known in the art, which utilize fluorescent tubes as an illumination source plus a CCD image scanner. One such scanning device is set forth in U.S. Pat. No. 3,867,569 to Watson which discloses a compact flatbed page scanner for a fascimile transmission system. The disclosed scanner utilizes a linear charge coupled imaging device for both light detection and electronic scanning across the width of the page, with the scanner system being illuminated by a fluorescent tube. There is no teaching in the Watson patent, however, relative to the use of an exposure controlled network, let alone an exposure controlled network fabricated on the scanner array itself.
U.S. Pat. No. 3,952,144 to Kolker discloses a method and apparatus for automatic background and contrast control in a fascimile scanning system. The fascimile transmitter makes a preliminary calibrating scan in which it sequentially scans a known reference indicia, for example, a black area and a white area, and the automatic background and contrast control unit fully automatically stores a first sample of the uncorrected video signal when the known black area is being scanned and stores a second sample of the uncorrected video signal when the known white area is being scanned. During subsequent scanning and transmission of the document, the automatic background and contrast control unit continually produces voltages representing the previously stored black and white samples and uses these voltages to continuously correct the video signals received during the subsequent scanning of the document. The Kolker patent teaches the use of a reference indicia, such as a black and white calibration strip, for exposure control units in which an electronic representation of the black and white samples is stored prior to the scanning of the document itself, with the stored reference values subsequently being used to correct for background variations during the subsequent scan of the document. There is, however, no teaching in the Kolker patent of the use of a CCD imaging system in which the exposure control is accomplished in the scanner chip itself, on a line by line basis concurrent with the scanning of the document.
U.S. Pat. No. 3,800,079 to McNeil et al discloses a compensation network for a scanning system, wherein a sensitivity profile of photo diodes forming a self scanned photo diode array operating in the charged storage mode is obtained as the array scans a standard background or white level while in a nonoperational or write mode. The serial output of the photo diode array is integrated and a predetermined gray level is subtracted to limit the resultant signal to the range affected by photo diode sensitivity variations and illumination non-uniformities. The resultant signal is digitized and stored under control of a write control signal. Compensation subsequently takes place in an operational mode. The stored digital signals are converted to an analog signal and the gray level is added thereto to correct the serial video information during an operating mode. The McNeil et al patent discloses a system somewhat similar to the Kolker patent, in which a background level is initially scanned and then stored and is subsequently used as a reference level as the document itself is scanned. Again, there is no mention of a charged transfer device array, let alone a line by line, concurrent exposure control correction of the scanned document by a reference network.
According to the present invention, a document scanner array is disclosed which compensates for temporal changes in a light source and other elements in the optical path, on a line by line basis through the use of a reference indicia which is scanned concurrent with the scanning of the document, rather than first scanning a reference indicia, storing a signal indicative thereof and then using the stored signal to modify the subsequently generated signals during the scanning of the document itself.